Refractory hard metal composite cathode aluminum reduction cell

ABSTRACT

The present invention is directed to cathodes of fused refractory hard metal alloy composited with certain bonding agents and their use, replacing the conventional carbon lining of an aluminum reduction cell.

United States Patent Holliday May 9, 1972 [54] REFRACTORY HARD lVIETALReferences Cited COMPOSITE CATHODE ALUMINUM UNITED STATES PATENTSREDUCTION CELL 3,400,061 9/1968 Lewis et a] ..204/67 [72] Inventor:Robin D. Holliday, New South Wales, 3,442,787 1969 Landfum 61 R XAustralia 3,502,553 3/1970 Gruber ..204/243 R X 3,514,520 5/1970Bacchiega et al. ..204/243 R X [73] Assigneez Olin Mathieson ChemicalCorporation Primary Examiner-John H. Mack [22] led. May 1969 AssistantExaminerD. R. Valentine [21] App]. No.: 822,395 Attorney-Richard S.Strickler, Robert H. Bachman, Donald R. Motsko and Thomas P. ODay [52]U.S. Cl. I ..204/67, 204/243 R, 204/291 [57] ABSTRACT [51] Int. Cl....C22d 3/12, C22d 3/02, B0lk 3/06 [58] Field of Search ..206/67,243-247, The Premt invention is directed cathodes of fused refrac-206/291 tory hard metal alloy composited with certain bonding agents andtheir use, replacing the conventional carbon lining of an aluminumreduction cell.

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ROB/N l2 HOLL /DAY 5 BY kj/ ZMMM ATTORNEY REFRACTORY HARD METALCOMPOSITE CATHODE ALUMINUM REDUCTION CELL Application of refractory hardmetals such as TiB -TiC alloys in aluminum reduction cells has beenpreviously considered to require structural members such as rods, bars,etc. Successful use of such members requires bodies having high-strengthand high density. Such bodies have been previously considered to befabricable only by hot-pressing.

It has proved extremely difficult to achieve the necessary high level ofmechanical properties in conjunction with the stringent control ofchemical composition essential for avoiding specific severe forms ofcorrosion.

Such materials have previously been considered for use in reductioncells, but only in the form of self-bonded, hotpressed shapes.Application has been restricted because of the difficulty and expenseinvolved in fabricating such shapes to meet the necessary stringentmechanical and composition requirements. See French Pat. No. 1,311,473.

According to the present invention it has been found that cathodes madeof fused refractory hard metal composited with certain bonding agentscan be used to replace the conventional carbon lining of an aluminumreduction cell.

Such composite cathodes combine the advantages of conventional carbonlinings (structural integrity and relative cheapness) with specificbasic property improvements conferred by the presence of a refractoryhard metal phase. These improvements are l) wettability by moltenaluminum, (2)

low overvoltage for aluminum deposition from cryolite, (3)

extremely low solubility in molten aluminum-cryolite systems, (4)freedom from penetration by aluminum-cryolite, when prepared in such away as to minimize deleterious impurity effects and (5) good electricalconductivity.

The requirements of composition control are met in the present inventionby employing refractory hard metals, preferably TiB -TiC alloys, whichhave been purified by arc melting under controlled conditions, asdescribed in my copending application, Ser. No. 822,705, filed May 7,1969. The amount of TiC in the alloy may vary over a wide range, forexample, from to 50 percent by weight.

This combination of properties makes it possible to use such compositesas solid monolithic cathodes on which Al ions may be deposited directly.If the aluminum is allowed to run off as it deposits and is collected ina suitable receptacle in the cell, then many disadvantages of the Hallcell system are by-passed.

The cathodes of the present invention make possible an enormousimprovement in the performance of reduction cells. Specifically, a 50percent reduction in anode-cathode distance is feasible when the moltenaluminum pool which now functions as cathode is replaced by a solid,wettable cathode block. Such a reduction in anode-to-cathode distanceproportionately reduces the interelectrode voltage drop. The voltagesavings may be utilized to obtain an increase of approximately 40percent in current density, with an equivalent increase in output ofmetal from the cell.

Refractory hard metals and alloys prepared in this way may be used asstructural members, if desired, but the particular purpose of thepresent invention is to reveal their application in the form ofcomposite bodies. For this purpose it is no longer necessary to considerlarge, self-bonded structural shapes of high mechanical integrity.Instead, fractured lumps, pieces and even small particles of refractoryhard metal alloys may be employed. The refractory hard metals may becomposited with bonding agents, such as graphite with pitch or othercarbonaceous binders. For such use, adherence to strict specificationsfor mechanical strength and impact strength is no longer necessary sincemechanical support is provided by the carbon, or other electricallyconducting matrix. The process of fabricating the composite bodies maybe similar to that for manufacture of carbon anodes, both in techniqueand in costs.

Thus, particulate refractory hard material ranging from pass 100 mesh toabove 4 mesh on the Tyler screen system may be prepared by casting, forexample, an alloy of TiB with 40 percent TiC in arc fumaces of knownconstruction equipped to directly produce the desired range of particlesizes by suitably dispersing and chilling the melt.

The resulting particulate grog may then be blended with graphite powderof the same general particle size distribution range in conventionalmixing apparatus.

The proportion of carbon or graphite to refractory hard metal grog canvary widely, over the range 2 to 50 volume percent refractory hardmetal. When the size of the RHM particles and the graphite or carbonparticles is essentially the same, it can be'shown that the percentageof exposed RHM surface area in the resulting carbon-bonded compositeequals the volume percentage of the refractory hard metal component. Tominimize material costs, the proportion of RHM to carbon should be theminimum needed to impart the desired electrochemical properties to thecomposite.

After addition and blending of pitch or other form of binder is carriedout, the resulting blend can be cold pressed into green bodies. Thesebodies may then be sintered using, for example, conditions similar tothose adopted in preparing anode carbon blocks for the Hall process. Inessence, the conditions provide for a gradual increase of temperatureover an extended time period until a maximum near l,100 C is reached.This brake is followed by a slow cool.

The method of fabricating composites of this type is, of course, notrestricted to a blending-sintering type operation. Hot-pressing of anarc melted refractory hard metal alloy-carbon mixture may be employed.Altemately, an alloy of refractory hard metal with carbon can beproduced directly in the arc furnace. Further, hot isostatic compactioncan be used in apparatus conventional for this process.

The resulting carbonRHM composites show a particularly desirablecombination of properties. The refractory hard metal component functionsas an equipotential surface on which deposition of aluminum ions occurspreferentially, i.e., the refractory hard metal forms theelectrochemically active I part of the composite system. The carbonmatrix provides mechanical support for the wettable RHM surface, and inaddition, permits electricity to be conducted from the RHM dominatedsurface to current collector bars such as are now used with conventionalcarbon linings.

It is postulated that the carbon fraction of the surface of thecomposite electrode becomes protected by an aluminum carbide layer. Thehigh overvoltage for aluminum and sodium deposition and the inherenthigh electrical resistivity of the aluminum carbide cooperate to make itimmune from further deterioration by aluminum or sodium once aprotective layer forms. This'aluminum carbide layer functions somewhatlike the corrosion resisting oxide films on stainless alloys.

The invention will be better understood from referring to the drawings.

FIG. 1 is a schematic view showing the refractory hardmetal-carbon-composite.

FIG. 2 is a view of another embodiment of the invention showing largechunks of refractory hard metal.

FIG. 3 is a perspective view of another embodiment of the invention andFIG. 3A is a typical section through FIG. 3.

FIG. 4 is a view of another embodiment of the invention.

FIG. 5 is a sectional view of an electrolyte cell according to thepresent invention.

The system of the present invention is depicted in HO. 1. RHM fused-castpurified alloy particulate grog, having'an electrochemically activesurface is shown at 10. The surface layer 11, believed to be of Al Cmakes this region electrochemically inert and stable with respect toaluminumcryolite. The supporting and conducting carbon matrix is shownat 12.

It is not necessary that composites be formed using only particles ofRHM grog. In another embodiment of the invention shown in FIG. 2, largechunks of RHM alloy 20, ranging from one-fourth inch to several inchesaverage diameter, may be embedded in a supporting carbon matrix.

In another type of composite shown in FIG. 3 and 3A, large chunks 30 ofREM alloy are cast to develop at least one reasonably flat side, and setin a conducting matrix to form a flagstone type surface. The surfacelayer 31 is shown in FIG.

It is not necessary that choice for the supporting matrix be restrictedto carbon and graphite. lndeed, aluminum carbide may be used as a fillermaterial. This is particularly suitable when relatively large chunks ofREM alloy are used in the flagstone type cathode. Moreover, either pureAl,C old reacted reduction cell linings, or Al C NaFAlF mixture may beused.

Preferably, as shown in FIG. 4, a cathode of this type will consistbasically of three components: (I) relatively large chunks of arc-meltedRHM alloy 40, (2) a powdered filler 41 such as ALC or similar materialbetween the refractory hard metal chunks and, (3) a lower conductivelayer 42, preferably carbon or graphite, forming a substrate layerproviding electrical contact between the RHM surface and currentcollector bars of the cell.

A three-component composite of this type would be fabricated, forexample, by cold-pressing followed by sintering, conditions being chosento promote formation of a solid bond between aluminum carbide particlesand between aluminum carbide and carbon. 7

It is also possible to achieve a satisfactory filler for a compositecathode by using aluminum carbide alloyed with materials such as TiC byarc welding.

Such a fusion process yields a mixed valency semi-conductor with a muchhigher electrical conductivity than aluminum carbide. ll: this case, useof a third component such as carbon is superfluous, and may be dispensedwith.

In addition to carbon and aluminum carbide, other materials of therefractory hard metal class such as TiC and TiN or their alloys are alsosuitable for bonding the electrochemically active solid fused chunks ofRHM alloy.

Clearly, there is no restriction on the size of composite shapes otherthan that imposed by size limits of apparatus needed for fabrication.For example, blocks may be fabricated in sizes of conventional bricks.Bonding of such bricks can be accomplished by means of conventionalcarbon pastes like those now used in reduction cells.

Also, a part of this invention is a reduction cell designedparticularlysuitable for using the composite cathodes of the typehereinbefore described.

As shown in FlG. such a reduction furnace shown generally at 50 maycomprise one or more carbon anodes such as 51 and roof or enclosure 53.A cell gas space 54 is found above the electrolyte 55. The RHM compositedry cathodes 56 are placed below the lower surface 51A of the anodes.

The sloped electrode surfaces 56A permit run-off of aluminum intoaccumulating chamber 57 and directional flow of anode gas to gas space54.

The carbon supporting matrix for the RHM composite is shown at 58. Thesteel cathode current collector bars 59 are embedded into insulation 52and supporting matrix 58.

As shown in FIG. 5, the composite cathodes may be used in cells wherethe molten aluminum is drained from the sloping surface 56A as it isdeposited, and accumulated in collection chamber 57 within the cell. Therequired slope of the wettable refractory hard metal surface is notlarge; a gradient of 1 to 10 is normally all that is needed to secureadequate removal of molten aluminum as it deposits. Obviously, however,a steeper slope can be used, such as the one shown in FIG. 5.

Such a design has the advantage of using an anode system identical tothat presently used in present commercially operated prebake cells.

The composite cathode materials of this invention are not limited toinstallations in a near horizontal position. In one embodiment of theinvention, composite cathodes may be appropriately shaped for use in avertical arrangement, either in cells using conventional cryoliticelectrolytes, or in cells for electrolytic reduction of aluminumchloride.

This invention may be embodied in other forms or carried l. A processfor producing aluminum comprising providing.

an electrolyte in a cell with aluminum dissolved therein, providing atleast one anode in said cell, providing at least one cathode in saidcell, said cathode comprising refractory hard metal imbedded in anelectrically conductive matrix said matrix selected from the groupconsisting of aluminum carbide and an arc melted material comprisingaluminum carbide and titanium carbide wherein the particles of said hardmetal are relatively larger than the particles of said matrix, passingelectrical current between said anode and said cathode causing saidalumina to react yielding aluminum ions, said aluminum ions depositingdirectly on said cathode.

2. A process according to claim 1 in which metallic aluminum runs offsaid cathode and is collected at a point other than at said cathode.

3. A process according to claim 2 in which said aluminum flows bygravity from said cathode to a separate collection receptacle withinsaid cell.

4. A process according to claim 1 wherein said refractory hard meta]comprises an arc melted material comprising titanium boride and titaniumcarbide.

5. A process according to claim 1 wherein said matrix comprises aluminumcarbide. v

6. A process according to claim 1 wherein said refractory hard metal isat least 0.25 inch average diameter.

7. A process according to claim 1 wherein said matrix comprises an arcmelted material comprising aluminum carbide and titanium carbide.

8. Apparatus for producing aluminum comprising means for confining amolten electrolyte containing alumina, at least one anode contactingsaid electrolyte, at least one cathode contacting said electrolyte, saidcathode comprising a composite of refractory hard metal imbedded in anelectrically conductive matrix said matrix selected from the groupconsisting of aluminum carbide and an arc melted material comprisingaluminum carbide and titanium carbide wherein the particles of said hardmaterial are relatively larger than the particles of said matrix, meansfor causing electrical current flow between said anode and said cathodewhereby aluminum ions are deposited on said cathode.

9. Apparatus according to claim 8 in which means are provided forcontinually removing and collecting metallic aluminum from said cathode.

10. Apparatus according to claim 9 in which the means for removingaluminum from said cathode is means which support said cathode on anincline.

11. Apparatus according to claim 8 in which said means for collecting isa chamber within said cell. i

12. An apparatus according to claim 8 wherein said refractory hard metalcomprises an arc melted material comprising titanium boride and titaniumcarbide.

13. An apparatus according to claim 8 wherein said matrix comprisesaluminum carbide.

14. An apparatus according to claim 8 wherein said refractory hard metalis at least 0.25 inch average diameter.

15. An apparatus according to claim 8 wherein said matrix comprises anarc melted material comprising aluminum carbide and titanium carbide.

2. A process according to claim 1 in which metallic aluminum runs off said cathode and is collected at a point other than at said cathode.
 3. A process according to claim 2 in which said aluminum flows by gravity from said cathode to a separate collection receptacle within said cell.
 4. A process according to claim 1 wherein said refractory hard metal comprises an arc melted material comprising titanium boride and titanium carbide.
 5. A process according to claim 1 wherein said matrix comprises aluminum carbide.
 6. A process according to claim 1 wherein said refractory hard metal is at least 0.25 inch average diameter.
 7. A process according to claim 1 wherein said matrix comprises an arc melted material comprising aluminum carbide and titanium carbide.
 8. Apparatus for producing aluminum comprising means for confining a molten electrolyte containing alumina, at least one anode contacting said electrolyte, at least one cathode contacting said electrolyte, said cathode comprising a composite of refractory hard metal imbedded in an electrically conductive matrix said matrix selected from the group consisting of aluminum carbide and an arc melted material comprising aluminum carbide and titanium carbide wherein the particles of said hard material are relatively larger than the particles of said matrix, means for causing electrical cUrrent flow between said anode and said cathode whereby aluminum ions are deposited on said cathode.
 9. Apparatus according to claim 8 in which means are provided for continually removing and collecting metallic aluminum from said cathode.
 10. Apparatus according to claim 9 in which the means for removing aluminum from said cathode is means which support said cathode on an incline.
 11. Apparatus according to claim 8 in which said means for collecting is a chamber within said cell.
 12. An apparatus according to claim 8 wherein said refractory hard metal comprises an arc melted material comprising titanium boride and titanium carbide.
 13. An apparatus according to claim 8 wherein said matrix comprises aluminum carbide.
 14. An apparatus according to claim 8 wherein said refractory hard metal is at least 0.25 inch average diameter.
 15. An apparatus according to claim 8 wherein said matrix comprises an arc melted material comprising aluminum carbide and titanium carbide. 